Owing to the increase in pubic demand for pressure sensitive postage stamps, bulk packaging of these stamps has become highly desirable, even essential. Such stamps have heretofore been available in sheets of 10 to 20 stamps held on a removable silicone coated liner, a form that does not lend itself to packaging in bulk. In addition, the silicone-containing liner material poses an environmental burden.
A model bulk packaging configuration for stamps is the currently available roll of 100 conventional stamps: storage space is minimized and the roll is used in its entirety. In order to configure pressure sensitive stamps similarly, however, the image-bearing surface of each stamp must be coated with a release agent which reversibly binds to the pressure sensitive adhesive on the opposing surface of another stamp to hold the roll in place during storage. The release agent easily separates from the adhesive layer as the roll unwinds.
Release agents have been used for many years, and composed of a variety of chemical formulations: urethanes, silicone copolymers, acrylates, acrylate copolymers, fluorocarbon polyamides, some water-borne and other dispersed by organic solvent. The particular properties of each release agent is also highly dependent upon the thickness of deposition and the porosity of the chosen substrate material.
Postage stamps have certain distinct features which impose several restrictions on the selection of a release agent for use with a linerless roll of pressure sensitive stamps. These restrictions result from unique postal handling procedures, including the used of cancellation marks atop the image-bearing surface of each posted stamp. For example, a suitable release agent must adhere securely to the adhesive material, yet be able to separate cleanly from it. On the other hand, the release agent must adhere permanently to the image-bearing surface of each stamp, remaining stable over a wide range of temperatures and humidities. Furthermore, the release coating must be transparent to reveal clearly both the stamp image and the phosphorescence of the stamp paper. Finally, the release coating must accept and retain a legible cancellation mark of glycol-based ink.
A copending application by Birkholz (Ser. No. 08/615,442), pending discloses linerless pressure sensitive postage stamp embodiments utilizing a release agent comprising a silicone-urea block copolymer with calcium carbonate dispersed therein at the level of 3 to 10 percent by weight. The block copolymer, having a preferred average molecular weight of about 4780, is disclosed and claimed in Tushaus, et al. (U.S. Pat. No. 5,290,615). The release agent is deposited upon the stamp paper as a dispersion in isopropyl alcohol, followed by drying to remove the alcohol.
While that formulation of release agent proved adequate for current postal procedures, even further reduction of the drying time for cancellation ink is highly desirable. Thus, an object of this invention was to optimize the formulation of the release agent, especially with regard to drying time of a glycol-based cancellation ink.